Strange behavior during error recovery

[Enrico Gregorio]

If one tries the following test file

\s^^?E

with any plain TeX engine (tex, pdftex, xetex, luatex),
pressing 1 after the “undefined control sequence”
error yields

! Undefined control sequence.
l.1 \s
      ^^?H
? 1
! Text line contains an invalid character.
l.1 \s^^?
         H
?

Now after pressing q, we get

OK, entering \batchmode

but nothing happens until return is pressed again.

If r is pressed instead of q, we get

! Undefined control sequence.
l.1 \s
      ^^?H
? 1
! Text line contains an invalid character.
l.1 \s^^?
         H
? r
OK, entering \nonstopmode...
l.1 \s^^?H

?

So the next (valid) token is indeed ignored, but the
messages are at least confusing. After hitting return
again, the message is

?
)
! Emergency stop.

so indeed \nonstopmode has been turned on.


I get no segmentation fault, like the poster at
https://tex.stackexchange.com/q/551313/4427
seems to get (macOS of unknown version).

I’m running TL 2020 on macOS 10.15 Catalina. The behavior is
the same with any engine from TL 2012 (the earlier version I
have on this machine). I don’t know whether it’s the same on
other platforms, but it’s probably worth investigating to see
if it’s something to be corrected in TeX itself or if it is
just a platform-specific issue.

Ciao
Enrico